System and method for controlling a power supply during discontinuous conduction mode

ABSTRACT

In accordance with an embodiment, a controller for a switched mode power supply includes an average current comparator that determines whether an average current within the switched mode power supply is below a current threshold, and a switch signal generation circuit coupled to the average current comparator having switch signal outputs configured to be coupled to a switching circuit of the switched mode power supply. The switch signal generation circuit produces a first switching pattern in a first mode of operation and produces a second switching pattern a second mode of operation. When the average current comparator determines that the average current is below the current threshold, the switch signal generation circuit is operated in a first mode, and when the average current comparator determines that the average current is not below the current threshold, the switch signal generation circuit is operated in a second mode.

TECHNICAL FIELD

The present invention relates generally to electronic circuits and powersupplies, and, in particular embodiments, to a system and method forcontrolling a power supply.

BACKGROUND

Power supply systems are pervasive in many electronic applications fromcomputers to automobiles. Generally, voltages within a power supplysystem are generated by performing a DC-DC, DC-AC, and/or AC-DCconversion by operating a switch loaded with an inductor or transformer.One class of such systems includes switched mode power supplies (SMPS).An SMPS is usually more efficient than other types of power conversionsystems because power conversion is performed by controlled charging anddischarging of the inductor or transformer and reduces energy lost dueto power dissipation across resistive voltage drops.

An SMPS usually includes at least one switch and an inductor ortransformer. Some specific topologies include buck converters, boostconverters, and flyback converters, among others. A control circuit iscommonly used to open and close the switch to charge and discharge theinductor. In some applications, the current supplied to the load and/orthe voltage supplied to the load is controlled via a feedback loop.

In some power supply applications, a switched mode power supply may beoperated in two different modes: a continuous conduction mode (CCM) anda discontinuous conduction mode (DCM). During CCM, the switch(es) may beoperated to continually charge or discharge the inductor. During DCM,the switch(es) may be operated so as to limit the amount of negativecurrent flowing in the inductor during each cycle in order to improveefficiency. In battery charging applications, CCM may be used during themiddle of the charging cycle and DCM may be used near the end of acharge cycle. As the load voltage or battery charge nears the target orsupply voltage, the power supply may be operated in DCM in order toreduce the charge removed from the load or battery by negative currents.Such approaches are common in charging applications for energyefficiency and to prevent battery damage.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a controller for a switched mode powersupply includes an average current comparator and a switch signalgeneration circuit coupled to the average current comparator. Theaverage current comparator determines whether an average current withinthe switched mode power supply is below a current threshold. Included inthe switch signal generation circuit are switch signal outputs that maybe coupled to a switching circuit of the switched mode power supply. Theswitch signal generation circuit produces a first switching pattern atthe switch signal outputs in a first mode of operation and produces asecond switching pattern at the switch signal outputs in a second modeof operation. When the average current comparator determines that theaverage current is below the current threshold, the switch signalgeneration circuit is operated in a first mode, and when the averagecurrent comparator determines that the average current is not below thecurrent threshold, the switch signal generation circuit is operated in asecond mode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIGS. 1a-b illustrate a block diagram of an embodiment switching powersupply system and control signal waveforms for an embodiment powersupply controller;

FIG. 2 illustrates a schematic of an embodiment switching power supplycircuit;

FIG. 3 illustrates a block diagram of an embodiment power supply system;

FIG. 4 illustrates a waveform diagram of an embodiment power supplysystem;

FIG. 5 illustrates a waveform diagram of an embodiment power supplysystem;

FIG. 6 illustrates a schematic of an embodiment current comparatorcircuit;

FIG. 7 illustrates a schematic of an embodiment current comparatorcircuit;

FIGS. 8a-b illustrate block diagrams of embodiment methods of operatinga power supply system;

FIG. 9 illustrates a waveform of an embodiment system.

FIG. 10 illustrates a block diagram of an embodiment method of operatinga switching power supply circuit;

FIG. 11 illustrates a block diagram of another embodiment method ofoperating a switching power supply circuit; and

FIG. 12 illustrates a block diagram of an embodiment calculation block.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the various embodimentsdescribed herein are applicable in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use various embodiments, and should not be construed ina limited scope.

Description is made with respect to various embodiments in a specificcontext, namely in power converter circuits. Various embodimentsdescribed herein include AC-DC or DC-DC voltage converters, switchingpower supply systems and circuits, battery charging circuits, currentcomparator circuits, power supply controllers, and other convertercircuits. In other embodiments, aspects may also be applied to otherapplications involving any type of converter or load driving circuitsusing integrated or discrete components according to any fashion ofelectronics as known in the art.

According to embodiments described herein, a switching power supplysystem and method of operating the switching power supply systemincludes operating the switching power supply system in at least twomodes, including at least continuous conduction mode (CCM) anddiscontinuous conduction mode (DCM). CCM may include cyclically chargingand discharging an inductive element which is coupled to a load, and DCMmay include cyclically charging for a time, discharging for a time, andconnecting in high impedance for a time the inductive element. Invarious embodiments, the switching power supply system is configured toswitch from CCM to DCM once an average current flowing in the inductiveelement drops below a first threshold. This average current may bedetermined over the course of a number of switching cycles. Theswitching power supply system is also configured to, during DCM, predicta time at which the current flowing in the inductor becomes zero. Insome embodiments, this prediction is performed without making a directmeasurement of the inductor current. The various embodiments describedherein provide details of structures and operations according to variousembodiments of the invention.

FIG. 1a illustrates a block diagram of an embodiment switching powersupply system 100 with a controller 110, a current comparator 120, aswitching converter circuit 130, and a load 140. According to anembodiment, switching converter circuit 130 provides a positive voltageline 118 and a negative voltage line 116 to load 140. Switchingconverter circuit 130 receives an input voltage 112 of magnitude V_(in)and is referenced to node 115. In various embodiments, node 115 may be aground node or may be a reference voltage other than ground.

According to an embodiment, switching converter circuit 130 receives aswitching control signal 124 from controller 110 and provides feedbacksignal 128 to controller 110 and a measurement signal 126 to currentcomparator 120. In various embodiments, measurement signal 126 is avoltage and/or current measurement provided from measurement block 132and corresponds to a current supplied to load 140. Measurement signal126 is connected to current comparator 120, which compares measurementsignal 126 with a threshold value and provides a comparison signal 122to controller 110. In some embodiments, comparison signal 122 conveys acomparison result between the current supplied to load 140 and thethreshold value. In various embodiments, current comparator 120 mayperform a current comparison between a current supplied to load 140 anda threshold current.

In some embodiments, controller 110 may be a proportional integralderivative (PID) controller. Alternatively, controller 110 may beimplemented as a current controller 110 capable of receiving a currentmeasurement signal 126 directly. In such an embodiment, currentcomparator 120 may be omitted and a current comparison is performed incurrent controller 110. In further embodiments, current controller 110may be a peak current controller 110 that measures a peak currentsupplied to load 140 via current measurement signal 126. In otherembodiments, current controller 110 may be an average current controller110 that measures an average current supplied to load 140 via currentmeasurement signal 126.

Controller 110 receives both comparison signal 122 from currentcomparator 120 and feedback signal 128 from switching converter circuit130. According to various embodiments, feedback signal 128 includesinformation about a voltage across load 140, a current entering orexiting load 140, or both the voltage of load 140 and the currentthrough load 140. Although depicted as single lines, it should be notedthat the connections depicted in FIG. 1a illustrate connections and mayinclude single connections or multiple connections (such as bus lines).In an embodiment, controller 110 uses comparison signal 122 and feedbacksignal 128 to determine switching control signal 124, which is providedto switching converter circuit 130.

According to a specific embodiment, load 140 is a battery and powersupply system 100 is a battery charging system. According to a specificembodiment, load 140 is being charged to a voltage lower than inputvoltage 112 on line 114. In alternative embodiments, load 140 may becharged to about input voltage 112 (i.e. a voltage of about or load 140may be charged to a load not equal to V_(in). Charging of load 140 to afinal voltage is achieved by switching converter circuit 130. In theembodiment, switching converter circuit 130 contains an inductor 106 andswitches 102 and 104 connected to inductor 106. Switches 102 and 104 areopened and closed in order to apply a voltage input to inductor 106 tocharge and discharge inductor 106 and supply a current to load 140.According to a specific embodiment, switching converter circuitcomprises a switched mode power supply (SMPS). Switches 102 and 104 arecontrolled by switching control signal 124 received from controller 110.

In an embodiment, controller 110 determines a duty cycle for opening andclosing switches 102 and 104. Power supply system 100 may be operated intwo different modes of operation: continuous conduction mode (CCM) anddiscontinuous conduction mode (DCM). Controller 110 may determine theduty cycle of switches 102 and 104 differently in CCM and DCM. In someembodiments, controller 110 transitions from the CCM to DCM according tocomparison signal 122, which may be, for example, an enable signal fortransitioning from the first mode of operation to the second mode ofoperation. Comparison signal 122 may be enabled when current supplied toload 140 goes below a threshold value. Alternatively, comparison signal122 may be disabled when current supplied to load 140 goes below acertain value.

FIG. 1b illustrates embodiment control signal waveforms for a powersupply controller 110. Upper switch waveform 102 g is a control signalapplied to a control terminal of switch 102 and lower switch waveform104 g is a control signal applied to a control terminal of switch 104.According to the embodiment shown, if upper switch waveform 102 g ishigh, switch 102 is closed and is lower switch waveform 104 g is high,switch 104 is closed. Switches 102 and 104 are operated in two differentmodes indicated by mode of operation 138. When mode of operation 138 isCCM, either switch 102 or 104 is closed. From another perspective,during CCM inductor 106 is either charging through switch 102 ordischarging through switch 104. In the embodiment shown, when mode ofoperation 138 is CCM, upper switch waveform 102 g and lower switchwaveform 104 g are inverse of each other. According to an embodiment,controller 110 will determine a duty cycle for controlling switches 102and 104. In the embodiment shown, the switch control duty cycle forupper switch waveform 102 g and lower switch waveform 104 g is about50%, however, the actual duty cycle may vary during operationaccordingly to include various other duty cycles. In various specificembodiments, CCM may be referred to as the second mode operation. In analternative embodiment, CCM may be referred to as the first modeoperation.

With continued reference to FIG. 1b , after transition 142 controller110 switches power supply system 100 from operating in CCM to DCM asindicated by mode of operation 138. During DCM, upper switch waveform102 g and lower switch waveform 104 g exhibit a tristate or highimpedance configuration in regions 144 when both switches 102 and 104are open. In an example sequence of operation, upper switch waveform 102g goes high, closing switch 102 and charging inductor 106. Next, lowerswitch waveform 104 g goes high when upper switch waveform 102 g goeslow, thus closing switch 104, opening switch 102, and discharginginductor 106. Finally, lower switch waveform 104 g goes low as upperswitch waveform 102 g remains low during a high impedance period inregions 144 when inductor 106 is not connected to either a charging nodeor a discharging node. According to embodiments described herein, a timeto open both switches 102 and 104 in order to enter the high impedanceperiod in regions 144 is determined such that when both switches areopened, the current in inductor 106 is close to zero. In variousembodiments, an inductor current zero crossing may be predicted duringDCM as part of determining the time to enter high impedance mode. Acontroller, such as controller 110, may be used to predict the inductorcurrent zero crossing during DCM. In various specific embodiments, DCMmay be referred to as the first mode of operation. In an alternativeembodiment, DCM may be referred to as the second mode of operation.

FIG. 2 illustrates a schematic of an embodiment switching power supplycircuit 200 having a first switch 202, a second switch 204, and aninductor 206. Switching power supply circuit 200 may correspond to anembodiment switching converter circuit 130 as described with referenceto FIGS. 1a-b . In an example embodiment, when switch 202 is closed,forming a conducting path between input node 212 and node 203, currentflows into the inductor 206 from the input node 212. Switch 204 may beopen, or non-conducting, when switch 202 is closed, or conducting. Next,switch 202 is opened and switch 204 is closed. Characteristic of aninductor, current continues to flow through inductor 206, resistor 232,and into load 240 when switch 204 is closed. Because inductor 206 iscoupled to reference node 215 and not to a supply or input node (such as212) when switch 204 is closed and switch 202 is open, current flowingthrough inductor 206 may decrease in an approximately linear fashion.

In the embodiment shown, a control signal coupled to control terminal222 opens and closes switch 202 and a control signal coupled to controlterminal 224 opens and closes switch 204. When switch 202 is closed andswitch 204 is open, current flowing through inductor 206 increases. Onthe other hand, when switch 204 is closed and switch 202 is open,current flowing into inductor 206 decreases. In some embodiments,opening and closing switches 202 and 204 supplies a current throughinductor 206 to load 240. In an example embodiment, the current suppliedto load 240 may be larger for a time period and smaller for another timeperiod. In an example involving battery charging, the current is largeduring regular charging and small during final charging (i.e. near anend of a charging cycle when the load/battery is almost fully charged).In the embodiment shown, a capacitor 208 connected in parallel with load240 acts as a low pass filter and reduces supply ripple across load 240.

In the embodiment depicted in FIG. 2, output signal 226 and outputsignal 228 may be connected as current or voltage measurement feedbackto control circuitry (not shown in FIG. 2). Resistors 232 and 234 mayhave known resistances, so that voltage differences V_(m1) and V_(m2)may be used to calculate the current supplied to or passing through load240 using Ohm's law.

In the embodiment depicted in FIG. 2, switch 202 is coupled betweeninput node 212, having a voltage signal V_(in), and node 203 and switch204 is coupled between reference node 215 and node 203. Inductor 206 iscoupled between node 203 and node 214. Resistor 232 is coupled toinductor 206 at node 214 and resistor 234 is coupled to reference node215. Capacitor 208 is coupled in parallel with load 240 between resistor232 and resistor 234 via nodes 218 and 216. A voltage difference acrossresistor 232 is provided as an output signal 226 depicted as V_(m1) anda voltage difference across resistor 234 is provided as an output signal228 depicted as V_(m2).

In some embodiments, when a small amount of current flows throughinductor 206, both switches 202 and 204 may be opened at the same time.In various embodiments, opening both switches at the same time is donein DCM and corresponds to a high impedance or tristate period. Openingboth switches 202 and 204 at the same time may prevent a negativecurrent through inductor 206 that would remove charge from load 240. Insome embodiments, the current supplied to load 240 by the switches 202and 204 and the inductor 206 is controlled in order to avoid negativecurrents that may discharge a battery connected as load 240. In someembodiments, the current supplied to load 240 by switches 202 and 204and inductor 206 may be controlled in order to avoid supplying too muchcurrent to prevent overcharging of a battery connected as load 240.

In various embodiments, both switches 202 and 204 are opened at the sametime when as little current as possible is flowing in inductor 206. Asdescribed with reference to FIG. 1b , an inductor current zero crossingmay be predicted to facilitate opening both switches 202 and 204 duringDCM in close vicinity to the current zero crossing. However, the smallamount of current still flowing in inductor 206 may continue to flowafter both switches 202 and 204 are opened. In some embodiments, currentmay flow through body diodes (not shown) of switches 202 and/or 204. Ina particular embodiment, a negative current from inductor 206 (i.e.flowing into switch 204) will continue flowing through the body diode ofswitch 204 as energy stored in inductor 206 is dissipated. In analternative embodiment, the small amount of current flowing in inductor206 may flow back and forth between the switch capacitance of switches204 and/or 202 and the inductor.

FIG. 3 illustrates a block diagram of an embodiment power supply system300 having a switching power supply in power stage 330 which supplies aload 340 by operating switches 302 and 304 in at least two modes ofoperation: CCM and DCM. In some embodiments, power supply system 300transitions from CCM to DCM when an average current supplied to load 340goes below a threshold stored, programmed, or provided to currentcomparator 320. In both CCM and DCM, controller 310 may determine a dutycycle for controlling switches 302 and 304 based on an error computationrelated to an output voltage or current supplied to load 340. In DCM,controller 310 may further predict an inductor current zero crossingthreshold for enabling tristate operation of switches 302 and 304.

In the embodiment shown in FIG. 3, power supply system 300 includes acontroller 310, a power stage 330, a current comparator 320, and adigital pulse width modulation (DPWM) block 336. Power stage 330includes a switching power supply circuit similar to switching powersupply circuit 200 shown in FIG. 2. Power stage 330 includes a high sideswitch 302 coupled between a voltage input node 312 and a switching node303 and a low side switch 304 coupled between switching node 303 andreference node 315. In some embodiments, reference node 315 is connectedto ground. Inductor 306 and resistor 332 are coupled in series betweenswitching node 303 and output node 318. Load 340 and capacitor 308 arecoupled in parallel between output node 318 and output node 316.Resistor 334 is coupled between reference node 315 and output node 316.

According to some embodiments, switch control signal 322 opens andcloses switch 302 and switch control signal 324 opens and closes switch304 in order to charge and discharge inductor 306. DPWM block 336provides control signals 322 and 324. In various embodiments, DPWM blockmay be a digital counter. Controller 310 provides a first thresholdsignal 335 and a second threshold signal 337 to DPWM block 336. In someembodiments, the first threshold signal 335 corresponds to a switchingduty cycle between high side switch 302 and low side switch 304. Thesecond threshold signal 337 may correspond to a zero crossing threshold.In some embodiments, the second threshold signal 337 is used to indicatewhen both switches 302 and 304 should be opened simultaneously.According to an embodiment, the second threshold signal 337 is usedduring DCM operation of a switching power supply. Operation of switches302 and 304 as well as the first and second threshold signals will bedescribed further below.

In some embodiments, current comparator 320 provides an enable signal321 to controller 310. Current comparator 320 may be a slow currentcomparator such as an averaging current comparator that averages currentover a number of cycles. Current comparator 320 receives a voltagesignal 326 from a voltage difference across resistor 332 and determinesenable signal 321 based on a current comparison between a thresholdcurrent and a current supplied to load 340 and passing through resistor332. In various embodiments, the threshold current used by currentcomparator 320 is programmable and may be changed in response to systemcharacteristics and requirements. In some embodiments, the currentsupplied to the load is determined using Ohm's law and voltage signal326. Controller 310 may operate DPWM block 336, and consequentlyswitches 302 and 304, in two different modes: CCM and DCM. In someembodiments, enable signal 321 may cause controller 310 to switchbetween CCM and DCM.

With further reference to FIG. 3, a current analog to digital (A2D)block 354 may provide a current measurement from load 340 to errorcomputation block 350. Current A2D block 354 receives an analog voltagesignal 328 corresponding to a voltage difference across resistor 334,converts the analog signal to a digital signal, and provides the digitalsignal corresponding to a voltage measurement from resistor 334 to errorcomputation block 350 (which can be used to calculate current throughresistor 334 based on Ohm's law). Voltage A2D block 352 receives analogvoltage signals 361 from load 340, converts the analog signal to adigital signal, and provides the digital signal corresponding to avoltage measurement from load 340 to error computation block 350. Errorcomputation block 350 computes an error signal 329 which is provided tocontroller 310. In various embodiments, controller 310 uses error signal329 to determine first and second threshold signals 335 and 337. In someembodiments, error signal 329 may be computed by error computation block350 with the aid of look-up tables 356 and 358. Look-up tables 356 and358 may include target currents and/or voltages and corresponding errorsignals based on current and voltage measurement signals from currentA2D block 354 and voltage A2D block 352. In some embodiments, look-uptables 356 and 358 are implemented as programmable registers. In otherembodiments, look-up tables 356 and 358 may be implemented using anytype of addressable memory.

FIG. 4 illustrates a waveform diagram of an embodiment power supplysystem showing inductor current signal 400, pulse width modulated (PWM)signal 410, tristate enable signal 420, and control mode 430 duringvarious modes of operation. The embodiment power supply system describedin FIGS. 4, 5, 8, and 9 may correspond to figures described herein (suchas FIGS. 1-3) or to other switching power supply systems. As shown inFIG. 4, inductor current signal 400 increases and decreases in responseto PWM signal 410. When PWM signal 410 is high, inductor current signal400 increases and when PWM signal 410 is low, inductor current signal400 decreases. Also shown are average inductor current signal 402,current threshold 404, and the zero current level 406.

During operation, an embodiment power supply system may operate indifferent modes. FIG. 4 depicts an embodiment having two primary modesof operation and a transition mode between the two primary modes.Control mode 430 depicts the two modes of operation and the transition:CCM, transition, and DCM. In an embodiment of CCM, tristate enablesignal 420 is not asserted and inductor current signal 400 may remainpositive (i.e. not below zero current level 406). As shown by theinductor current signal 400, the inductor current may become negative asthe current is decreasing during CCM. In some embodiments, a negativeinductor current is avoided by switching to DCM.

In various embodiments, average inductor current signal 402 is used toindicate when to change from CCM to DCM. Particularly, if averageinductor current 402 is below current threshold 404, the mode ofoperation may change. In one embodiment, the mode changes from CCM toDCM when average inductor current signal 402 is below current threshold404. In various embodiments, current threshold 404 is programmable. Atransition period or mode may be helpful in some embodiments. As shownin FIG. 4, when average inductor current signal 402 is below currentthreshold 404, control mode 430 changes from CCM to the CCM-DCMtransition mode. In some embodiments, the CCM-DCM transition modeincludes a set number of cycles. In a specific embodiment, the number ofcycles ranges from 1 to 128.

According to various embodiments, the CCM-DCM transition mode providestime for a controller to calculate a new duty-cycle for DCM for highside and low side switches and may provide time for a control loop tocause the inductor current to become more stable. In specificembodiments, the low side switch is turned off during CCM-DCM transitionmode, which may prevent negative current in the inductor. According tosome embodiments, when the low side switch is turned off, averageinductor current signal 402 may increase. In such an embodiment, thecontroller may maintain control mode 430 in CCM-DCM transition mode evenif average inductor current signal 402 rises above current threshold404. In some embodiments, the CCM-DCM transition mode may be very shortor omitted because the new duty cycle may be pre-calculated for aparticular system or the controller used may be very fast. After theCCM-DCM transition mode, control mode 430 may change to DCM.

In an embodiment, during the CCM-DCM transition mode, tristate enablesignal 420 is asserted when PWM signal 410 is low. Asserting tristateenable signal 420 may indicate that all switches connected to aninductor are open. According to various embodiments, tristate enablesignal 420 corresponds to turning off both a low side switch and a highside switch and does not necessarily include a separate physical controlsignal apart from the two control signals for the high and low sideswitches. For example, asserting tristate enable signal 420 may indicatethat switches 302 and 304 are both open simultaneously in FIG. 3.According to an alternative embodiment, tristate enable signal 420includes a physical control signal attached to the high and low sideswitches.

In some embodiments, during the CCM-DCM transition mode, inductorcurrent flows through a body diode of the low side transistor whentristate enable signal 420 is asserted. In an embodiment, during DCM,tristate enable signal 420 is asserted sometimes when PWM signal 410 islow. According to an embodiment, during DCM, tristate enable signal 420is asserted when inductor current signal 400 is zero or below zero. Insome embodiments, a point where inductor current signal 400 goes to zerois predicted and not directly measured in a control block when controlmode 430 is operating in DCM.

FIG. 5 illustrates a waveform diagram of an embodiment power supplysystem during DCM operation. A cycle is depicted during which PWM countsignal 500 is decreasing. In an embodiment, as PWM count signal 500reaches certain thresholds, different switches are opened or closed. Inan embodiment, high side control signal 522 is a control signal for aswitch coupled to an input voltage (such as switch 302 in FIG. 3) andlow side control signal 524 is a control signal for a switch coupled toa reference node (such as switch 304 in FIG. 3).

In the embodiment depicted in FIG. 5, control signal 522 is assertedduring a first time period T_HS (time high side), control signal 524 isasserted for a second time period T_LS (time low side), and enabletristate signal 520 is asserted for the remainder of a cycle period Tsw(i.e. Tsw−(T_HS+T_LS)). According to various embodiments, enabletristate signal 520 corresponds to a time period when control signals522 and 524 are both low (or disabled) and does not include a separatephysical control signal. In other embodiments, enable tristate signal520 may be implemented as a physical control signal that disables boththe high and low side switches.

T_HS may correspond to the time when PWM count signal 500 decreases fromits start to duty threshold 535, T_LS may correspond to the time whenPWM count signal 500 decreases from duty threshold 535 to zero crossingthreshold 537, and the remaining time corresponds to counting out therest of period Tsw. In other embodiments, an increasing counter may beimplemented that counts up to a duty threshold. According to theembodiment shown, time period T_HS may correspond to duty threshold 535which is computed based on an error signal determined through feedbackfrom a power supply load (such as error signal 329 in FIG. 3). Invarious embodiments, time period T_LS may correspond to zero crossingthreshold 537 which is estimated according to the following equation,

${T\_ LS} = {{T\_ HS}*( {\frac{V_{i\; n}}{V_{out}} - 1} )}$where V_(in) is a supplied input voltage to the power supply system, andV_(out) is a voltage on a load of the power supply system. In someembodiments, T_LS is calculated in a control block (such as controller310 in FIG. 3). In various embodiments, zero crossing threshold 537 isan estimated point when current flowing through an inductor to the loadreaches zero.

FIG. 6 illustrates a schematic of an embodiment current comparatorcircuit 600 attached across a sense resistor 602. As shown, currentcomparator circuit 600 includes an average current measurement circuit601 and a comparator 611. Inductor 606 is connected in series withresistor 602 between a first node 603 and a second node 604. In someembodiments, node 603 corresponds to switching node 303 in FIG. 3 andnode 604 corresponds to node 318 in FIG. 3. A current flowing throughresistor 602 causes a voltage across resistor 602 which determines aresponse of the current comparator circuit 600.

In the embodiment shown in FIG. 6, the voltage across resistor 602 isused as an input to an operational transconductance amplifier (OTA) 608.OTA 608 may convert the input voltage to an output current flowing intonode 618. Node 618 is connected to a positive terminal of differentialamplifier 610. The current flowing into node 618 may produce a voltageon node 618 which is compared by differential amplifier 610 to athreshold voltage supplied by threshold input 614 to a negative input ofdifferential amplifier 610. Output node 621 corresponds to a result ofthe comparison between the threshold voltage on input 614 and the outputof OTA 608. In the embodiment shown, capacitor 612 and resistor 616 areconnected in parallel between node 618 and a reference node. In variousembodiments, the reference may be ground. Capacitor 612 may preventvoltage spikes or jumps and resistor 616 may provide a current path to areference node. Capacitor 612 may also filter and/or average the voltagesignal on node 618. In alternative embodiments, average currentmeasurement circuit 601 and comparator 611 may be implemented usingother circuits and methods known in the art.

According to an embodiment, output node 621 may be used as an enablesignal for a controller (such as controller 310 and enable signal 321 inFIG. 3). In various embodiments, output node 621 may provide a signal toenable or disable a first or second operating mode within a controller.In some embodiments, V_(thresh) on input 614 is chosen as a thresholdvoltage that corresponds to a particular current value flowing throughinductor 606 and resistor 602. If current flowing through inductor 606and resistor 602 drops below a specific level corresponding to a voltageon node 618 less than V_(thresh), output node 621 may change sign. Invarious embodiments, output node 621 may produce an active high signalor an active low signal.

FIG. 7 illustrates a schematic of an embodiment current comparatorcircuit 700 having two transistors 708 and 710 coupled to a resistor702. Inductor 706 and resistor 702 are connected in series between afirst node 703 and a second node 704. A current source 712 may biastransistor 710 and a variable current source 714 may bias transistor708. Variable current source 714 is set to a threshold current I₂ whichmay be chosen to correspond to a specific current flowing throughresistor 702 and inductor 706. A current flowing through transistor 708is inversely proportional to the current flowing through resistor 702.As the current flowing through resistor 702 decreases (which also meansthat the current flowing through inductor 706 decreases), currentflowing through transistor 708 increases. When the current flowingthrough transistor 708 is above threshold current I2, a current flowsinto capacitor 716.

In some embodiments, capacitor 716 filters an input voltage of buffer718. Buffer 718 drives output 721 that toggles when the current flowingthrough resistor 702 and inductor 706 exceeds a programmable thresholddetermined by threshold current I2. In some embodiments, buffer 718 maybe an inverter as illustrated in FIG. 7. Output 721 may provide anactive high or an active low signal. According to a specific example,output 721 provides a digital output signal.

FIGS. 8a-b illustrate block diagrams of embodiment methods of operatinga power supply system. FIG. 8a illustrates an embodiment method 800 ofestimating an inductor current zero crossing in a switching powersupply. Method 800 includes first populating look-up tables in step 802.In some embodiments, step 802 may be omitted if relevant calculationsare performed in real-time, for example, using a processor. After step802, DCM is enabled in step 804, and an output voltage is measured instep 806, which may be a voltage across a load (such as loads 140, 240,and 340 in FIGS. 1-3). In an embodiment, the output voltage is a batteryvoltage during charging. After step 806, the output voltage is used instep 808 to look up a division result for V_(in)/V_(out) to be used inthe equation above. Using the division result from step 808 and the timeperiod T_HS, calculation of T_LS is completed in step 810 and finally isprovided to a switching block (such as DPWM block 336 in FIG. 3) in step812. In various embodiments, the switching block includes circuitry towait or count the time periods determined (T_HS, T_LS, and a remainder).The circuitry in the switching block may include a digital counter. Insome embodiments, T_LS may be recalculated with a newly measured outputvoltage by going back to step 806 as illustrated by arrow 814. Accordingto various embodiments, V_(out) and/or V_(in) may be measured once perswitching period and the calculation of T_LS and/or T_HS may beperformed during the switching period.

FIG. 8b illustrates an embodiment method 830 of controlling switches ina switching power supply during a mode of operation. In variousembodiments, the mode of operation is DCM. Method 830 includes a highside switch enable step 822, a low side switch enable step 824, and aswitch disable or tristate step 820. These steps can be continuallyrepeated during DCM as shown by arrow 826. According to someembodiments, step 822 includes enabling a high side switch (such asswitch 302 in FIG. 3) for time period T_HS, step 824 includes enabling alow side switch (such as switch 304 in FIG. 3) for time period T_LS, andstep 820 includes disabling both the high side switch and the low sideswitch. According to various embodiments, the transition from step 822to step 824 depends on a duty threshold and the transition from step 824to step 820 depends on a zero crossing threshold. In the variousembodiments, the zero crossing threshold may be digitally estimated.

FIG. 9 illustrates a waveform of an embodiment power supply systemduring operation. Waveform 910 depicts an embodiment current flowingthrough an inductor or inductive element in a switching power supplywith a high side switch and a low side switch. Waveform 920 depicts avoltage on a control node of the low side switch and waveform 930depicts a voltage on an intermediate node (such as node 303 in FIG. 3).In the embodiment shown, when the inductor current shown by waveform 910is increasing, the high side switch is closed (or conducting), as seenby the intermediate node voltage waveform going high, and the low sideswitch is open (or non-conducting). When the inductor current shown bywaveform 910 stops increasing and begins decreasing, the high sideswitch is opened and the low side switch is closed, as can be seen bywaveform 920 going high and waveform 930 going low. When the inductorcurrent shown by waveform 910 reaches zero, the low side switch isopened as shown by waveform 920 going low. When both the high sideswitch and the low side switch are open during time period 940, theinductor current shown in waveform 910 is prevented from going verynegative. In some embodiments, the inductor and switch capacitance ofthe low side transistor behave as an LC tank during time period 940, andthe current and voltage oscillate temporarily. In an embodiment, timeperiod 940 may correspond to a tristate phase during which both the highand low side switches are open.

FIG. 10 illustrates a block diagram of an embodiment method 1000 ofoperating a switching power supply circuit which includes asserting afirst switching command 1010, asserting a second switching command 1020,and asserting a third switching command 1030 for a switching powersupply. Method 1000 includes determining time 1040 that determines howlong to continue asserting the second switching command 1020 beforetransition 1024.

According to an embodiment, first switching command 1010 may includecharging inductor step 1012 during which an inductor in the switchingpower supply circuit is charged. Similarly, second switching command1020 may include discharging inductor step 1022 during which theinductor is discharged and third switching command 1030 may include highimpedance step 1032 during which the inductor is connected to a highimpedance node.

In some embodiments, operating counter step 1050 may include operating acounter or delay system to provide transition 1014, transition 1024, andtransition 1034 based on a first threshold, a second threshold, and acycle period. The first threshold may depend on an electrical propertyof the load (e.g. current or voltage), the second threshold may dependon a predicted inductor current zero crossing, and the cycle period maybe chosen based on system design characteristics.

According to an embodiment, method 1000 includes operating in DCM.Method 1000 may also include operating in CCM, during which transition1016 may be used to transition repeatedly between asserting the firstswitching command 1010 and asserting the second switching command 1020.In a specific instance, DCM omits transition 1016 and includestransition 1024 to asserting the third switching command 1030.

FIG. 11 illustrates block diagram of another embodiment method 1100 ofoperating a switching power supply circuit that includes three modes:CCM, transition mode, and DCM. As previously described herein, aswitching power supply circuit may be operated in CCM until an averagecurrent threshold is reached. When the average current threshold isreached, a controller may change the mode of operation of the switchingpower supply circuit from CCM to the transition mode. In someembodiments, if a load condition changes while the switching powersupply circuit is in the transition mode, the controller may change backto operating in CCM. A changed load condition may include a highercurrent demand or voltage requirement, for example. In some embodiments,after a specific number of cycles, the controller may change to operatethe switching power supply circuit in DCM. The number of cycles toremain in the transition mode may be adjustable and may take any valuebased on the system design and requirements.

According to various embodiments described herein, a controller for aswitched mode power supply includes an average current comparator and aswitch signal generation circuit coupled to the average currentcomparator. The average current comparator determines whether an averagecurrent within the switched mode power supply is below a currentthreshold. The switch signal generation circuit includes switch signaloutputs that may be coupled to a switching circuit of the switched modepower supply. The switch signal generation circuit produces a firstswitching pattern at the switch signal outputs in a first mode ofoperation and produces a second switching pattern at the switch signaloutputs in a second mode of operation. The switch signal generationcircuit is operated in a first mode when the average current comparatordetermines that the average current is below the current threshold andin a second mode when the average current comparator determines that theaverage current is not below the current threshold.

According to another embodiment, the switch signal generation circuit isfurther configured to produce a third switching pattern at the switchsignal outputs for a first number of switching cycles after the averagecurrent comparator determines that the average current is below thecurrent threshold prior to producing the first switching pattern. Insome embodiments, the first switching pattern is configured to operatethe switched mode power supply in a discontinuous conduction mode (DCM),the second switching pattern is configured to operate the switched modepower supply in a continuous conduction mode (CCM), and the thirdswitching pattern is configured to operate the switched mode powersupply in a CCM to DCM transition mode.

According to various embodiments, the second switching pattern includesrepeating cycles of alternatingly activating a first switch signaloutput and a second switch signal output, the first switching patternincludes repeating cycles of asserting a first switch state, followed byasserting a second switch state, followed by asserting a third switchstate, and the third switching pattern comprises repeating cycles ofasserting the first switch state and the third switch state. Assertingthe first switch state may include activating the first switch signaloutput and deactivating the second switch signal output, asserting thesecond switch state may include deactivating the first switch signaloutput and activating the second switch signal output, and asserting thethird switch state may include deactivating the first switch signaloutput and deactivating the second switch signal output.

In an embodiment, the controller determines a length of time forasserting the second switch state based on an input voltage and anoutput voltage of the switched mode power supply. The average currentcomparator may include an average current measurement circuit and acomparator.

According to an embodiment, a controller for a switched mode powersupply includes a pulse width modulator configured to be coupled to aswitching circuit of the switched mode power supply. In an embodiment,in a first mode, the pulse width modulator cyclically asserts a firstswitching command, a second switching command, and a third switchingcommand to the switching circuit; and determines a length of time of thesecond switching command based on an input voltage and an output voltageof the switched mode power supply.

In various embodiments, the first switching command causes the switchingcircuit to charge an inductor coupled to an output port of the switchedmode power supply, the second switching command causes the switchingcircuit to discharge the inductor coupled to the output port of theswitched mode power supply, and the third switching command causes theswitching circuit to attain a high impedance state.

In some embodiments, the pulse width modulator includes a counter and afirst comparator coupled to an output of the counter. The pulse widthmodulator is configured to transition from asserting the secondswitching command to the third switching command when the firstcomparator determines that a value at the output of the counter crossesa first threshold. The first threshold is dependent on the input voltageand the output voltage of the switched mode power supply. In someembodiments, the counter includes a digital counter and the firstcomparator includes a digital comparator.

In some embodiments, the controller includes a second comparator coupledto an output of the counter, and the pulse width modulator transitionsfrom asserting the first switching command to the second switchingcommand when the second comparator determines that a value at the outputof the counter crosses a second threshold. The second threshold may bedetermined according to at least one of an output voltage and an outputcurrent of the switched mode power supply.

In various embodiments, the pulse width modulator asserts the firstswitching command by activating a first switch output signal anddeactivating a second switch output signal, asserts the second switchingcommand by deactivating a first switch output signal and activating asecond switch output signal, and asserts the third switching command bydeactivating the first switch output signal and deactivating the secondswitch output signal. In an embodiment, the controller is configured toactivate a tristate enable signal. Asserting the third switching commandmay include activating the tristate enable signal.

In various embodiments, the controller includes an average currentcomparator that determines whether an average current within theswitched mode power supply is below a current threshold. The pulse widthmodulator may operate in a second mode when the average currentcomparator indicates that the average current is not below the currentthreshold. In the second mode, the pulse width modulator may cyclicallyassert the first switching command and the second switching command tothe switching circuit. In some embodiments, the current threshold is anadjustable threshold. The first mode may include a discontinuousconduction mode (DCM) and the second mode may include a continuousconduction mode (CCM). In various embodiments, the controllertransitions from the second mode to the first mode when the averagecurrent comparator determines that the average current is below thecurrent threshold.

In various embodiments, the controller transitions from the second modeto a third mode of operation when the average current comparatordetermines that the average current is below the current threshold. Inthe third mode, the pulse width modulator cyclically asserts the firstswitching command and the third switching command to the switchingcircuit. The controller may also transition from the third mode to thefirst mode after a predetermined number of cycles of the third mode. Insome embodiments, the controller transitions from the third mode back tothe second mode if a load condition changes. The controller may alsotransition directly from the first mode to the second mode if a loadcondition changes.

According to various embodiments, a method of operating a switched modepower supply includes cyclically asserting a first switching command, asecond switching command, and a third switching command to a switchingcircuit. The method also includes determining a length of time of thesecond switching command based on an input voltage and an output voltageof the switched mode power supply.

In some embodiments, the method includes charging an inductor coupled toan output port of the switched mode power supply using the switchingcircuit when asserting the first switching command, discharging theinductor coupled to the output port of the switched mode power supplyusing the switching circuit when asserting the second switching command,and placing the switching circuit in a high impedance state whenasserting the third switching command.

In some embodiments, cyclically asserting includes operating a counterand transitioning from asserting the second switching command toasserting the third switching command when an output value of thecounter crosses a first threshold. The first threshold may be dependenton the input voltage and the output voltage of the switched mode powersupply. In an embodiment, cyclically asserting includes transitioningfrom asserting the first switching command to asserting the secondswitching command when an output value of the counter crosses a secondthreshold. The second threshold may be dependent on at least one of anoutput voltage or an output current of the switched mode power supply.In another embodiment, cyclically asserting further includestransitioning from asserting the third switching command to assertingthe first switching command when an output value of the counter crossesa third threshold equal to a period of the switched mode power supply.

An advantage of various embodiments described herein includes theability to implement DCM in a switching power supply system bypredicting the inductor current zero transition points without having tomeasure the inductor current at each cycle, thereby reducing the size,cost and power consumption of power converter systems. A furtheradvantage of some embodiments include the ability to accurately predictthe discharge time of an inductor, thereby reducing the magnitude ofnegative current during DCM. Further benefits of such embodimentsinclude higher efficiency.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

FIG. 12 illustrates a block diagram of an embodiment calculation block1200 including threshold calculation unit 1210. Threshold calculationunit 1210 calculates zero crossing threshold ZC_(thresh) based on inputvoltage V_(in) for an SMPS and output voltage V_(out) of the SMPS asdescribed hereinabove in reference to the other figures. Thresholdcalculation unit 1210 also calculates duty threshold D_(thresh) based onat least one of input voltage V_(in) for the SMPS and output voltageV_(out) of the SMPS as described hereinabove in reference to the otherfigures.

What is claimed is:
 1. A controller for a switched mode power supply,the controller comprising: an average current comparator configured todetermine whether an average current within the switched mode powersupply is below a current threshold; and a pulse width modulatorconfigured to be coupled to a switching circuit of the switched modepower supply, wherein, in a first mode, the pulse width modulator isconfigured to cyclically assert a first switching command, a secondswitching command, and a third switching command to the switchingcircuit; and calculate a length of time of the second switching commandbased on an input voltage and an output voltage of the switched modepower supply; and wherein the pulse width modulator is furtherconfigured to operate in a second mode when the average currentcomparator indicates that the average current is not below the currentthreshold, wherein, in the second mode, the pulse width modulator isconfigured to cyclically assert the first switching command and thesecond switching command to the switching circuit.
 2. The controller ofclaim 1, wherein: the first switching command is configured to cause theswitching circuit to charge an inductor coupled to an output port of theswitched mode power supply; the second switching command is configuredto cause the switching circuit to discharge the inductor coupled to theoutput port of the switched mode power supply; and the third switchingcommand is configured to cause the switching circuit to attain a highimpedance state.
 3. The controller of claim 1, wherein the pulse widthmodulator is configured to: assert the first switching command byactivating a first switch output signal and deactivating a second switchoutput signal; assert the second switching command by deactivating thefirst switch output signal and activating the second switch outputsignal; and assert the third switching command by deactivating the firstswitch output signal and deactivating the second switch output signal.4. The controller of claim 3, wherein the controller is furtherconfigured to activate a tristate enable signal, and wherein assertingthe third switching command comprises activating the tristate enablesignal.
 5. The controller of claim 1, wherein the current threshold isan adjustable threshold.
 6. The controller of claim 1, wherein the firstmode comprises a discontinuous conduction mode (DCM) and the second modecomprises a continuous conduction mode (CCM).
 7. The controller of claim1, wherein the controller is further configured to transition from thesecond mode to the first mode when the average current comparatordetermines that the average current is below the current threshold. 8.The controller of claim 1, wherein the controller is further configuredto: transition from the second mode to a third mode when the averagecurrent comparator determines that the average current is below thecurrent threshold, wherein, in the third mode, the pulse width modulatoris configured to cyclically assert the first switching command and thethird switching command to the switching circuit.
 9. The controller ofclaim 8, wherein the controller is further configured to transition fromthe third mode to the first mode after a predetermined number of cyclesof the third mode.
 10. The controller of claim 8, wherein the controlleris further configured to transition from the third mode back to thesecond mode if a load condition changes.
 11. The controller of claim 8,wherein the controller is further configured to transition directly fromthe first mode to the second mode if a load condition changes.
 12. Thecontroller of claim 1, wherein calculating the length of time of thesecond switching command comprises using a look-up table.
 13. Thecontroller of claim 1, wherein a length of time of the first switchingcommand is given by T1; the length of time of the second switchingcommand is given by T2; the input voltage is given by VIN; the outputvoltage is given by VOUT; and calculating the length of time of thesecond switching command comprises calculating using an equationT2=T1*(VIN/VOUT−1).
 14. A controller for a switched mode power supply,the controller comprising: a pulse width modulator configured to becoupled to a switching circuit of the switched mode power supply,wherein, in a first mode, the pulse width modulator is configured tocyclically assert a first switching command, a second switching command,and a third switching command to the switching circuit; and determine alength of time of the second switching command based on an input voltageand an output voltage of the switched mode power supply; a counter; anda first comparator coupled to an output of the counter, wherein thepulse width modulator is further configured to transition from assertingthe second switching command to the third switching command when thefirst comparator determines that a value at the output of the countercrosses a first threshold, and the first threshold is dependent on theinput voltage and the output voltage of the switched mode power supply.15. The controller of claim 14, wherein: the counter comprises a digitalcounter; and the first comparator comprises a digital comparator. 16.The controller of claim 14, further comprising a second comparatorcoupled to an output of the counter, wherein the pulse width modulatoris further configured to transition from asserting the first switchingcommand to the second switching command when the second comparatordetermines that a value at the output of the counter crosses a secondthreshold.
 17. The controller of claim 16, wherein the second thresholdis determined according to at least one of an output voltage and anoutput current of the switched mode power supply.
 18. A method ofoperating a switched mode power supply, the method comprising: comparingan average current within the switched mode power supply to a currentthreshold using an average current comparator; operating the switchedmode power supply in a first mode when the average current is below thecurrent threshold, the first mode comprising: cyclically asserting afirst switching command, a second switching command, and a thirdswitching command to a switching circuit; and calculating a length oftime of the second switching command based on an input voltage and anoutput voltage of the switched mode power supply; and operating theswitched mode power supply in a second mode when the average current isnot below the current threshold, the second mode comprising cyclicallyasserting the first switching command and the second switching commandto the switching circuit.
 19. The method of claim 18, furthercomprising: charging an inductor coupled to an output port of theswitched mode power supply using the switching circuit when assertingthe first switching command; discharging the inductor coupled to theoutput port of the switched mode power supply using the switchingcircuit when asserting the second switching command; and placing theswitching circuit in a high impedance state when asserting the thirdswitching command.
 20. The method of claim 18, wherein calculating thelength of time of the second switching command comprises using a look-uptable.
 21. The method of claim 18, wherein a length of time of the firstswitching command is given by T1; the length of time of the secondswitching command is given by T2; the input voltage is given by VIN; theoutput voltage is given by VOUT; and calculating the length of time ofthe second switching command comprises calculating using an equationT2=T1*(VIN/VOUT−1).
 22. A method of operating a switched mode powersupply, the method comprising: cyclically asserting a first switchingcommand, a second switching command, and a third switching command to aswitching circuit; and determining a length of time of the secondswitching command based on an input voltage and an output voltage of theswitched mode power supply, wherein cyclically asserting comprises:operating a counter; and transitioning from asserting the secondswitching command to asserting the third switching command when anoutput value of the counter crosses a first threshold, wherein the firstthreshold is dependent on the input voltage and the output voltage ofthe switched mode power supply.
 23. The method of claim 22, whereincyclically asserting further comprises: transitioning from asserting thefirst switching command to asserting the second switching command whenan output value of the counter crosses a second threshold, wherein thesecond threshold is dependent on at least one of an output voltage or anoutput current of the switched mode power supply.
 24. The method ofclaim 22, wherein cyclically asserting further comprises: transitioningfrom asserting the third switching command to asserting the firstswitching command when an output value of the counter crosses a thirdthreshold equal to a period of the switched mode power supply.